This invention relates to electrical resistance heaters and more particularly to such heaters formed in situ on metal substrates.
Electrically heated metal bodies have a wide variety of commercial and industrial uses, such as in thermostatic devices, thermal relays, time-delay relays, circuit breakers, etc. It is advantageous to have the electrically energized heater in good heat-exchange relation to the metal body, frequently a bimetal strip or disk, which changes its configuration as a function of temperature. Also, it is desirable to be able to supply such heater-metal units in various shapes and configurations at minimal expense. By providing a heater constituted by a relatively thin layer or coating applied on a surface area of the metal substrate to be heated, excellent heat transfer can be achieved. However, such heater layers are subjected to high temperatures for extended periods of time and in many applications must undergo repeated flexing. Epoxy resins mixed with graphite or other materials have been used for this purpose but at elevated temperatures, i.e., in the order of 200.degree.C. or higher, these materials tend to degrade and deteriorate and fail to provide stable resistance characteristics necessary to long-term reliable functioning. Electrical resistance heater tapes and films have been made of polyimide and polyamide-imide resin compositions containing carbon particles, as disclosed in U.S. Pat. Nos. 3,444,183, 3,563,916, Belgium Pat. No. 630,749 and Netherlands application Ser. No. 6,511,346. There remains, however, a need for heater-metal composite units which will reliably function at elevated temperatures and economically provide for convenient supply of electrical current to the unit and flow through desired paths.